deep-active-element.js

/**
 * Returns the deepest active element.
 *
 * @param {HTMLElement} element - The element to search.
 * @returns {!HTMLElement} - Active element
 */
export function deepActiveElement(element = document.activeElement) {
  let elementActive = element;
  
  // document.activeElement can be null
  // https://developer.mozilla.org/en-US/docs/Web/API/Document/activeElement
  // In IE 11, it can also be an object when operating in iframes.
  // In these cases, default it to document.body.
  if (!elementActive || elementActive instanceof Element === false) {
    elementActive = document.body;
  }

  if (elementActive.shadowRoot) {
    while (elementActive.shadowRoot && elementActive.shadowRoot.activeElement) {
      elementActive = elementActive.shadowRoot.activeElement;
    }
  }

  return elementActive;
}